Structure-Property-Process Relationships of Melt-Blown PVDF: A Potential Li-Ion Battery Separator

Abstract

While significant research efforts have focused on the negative and positive electrode materials in rechargeable Lithium-ion (Li-ion) batteries, battery separators have only recently received more consideration from the scientific community. The separator plays a critical role in Li-ion batteries by preventing physical contact between the positive and negative electrodes while permitting efficient ionic transport across the separator. There are four major types of separators: microporous polymeric membranes, nonwoven polymeric mats, gel-polymer electrolytes and composite membranes. Relative to the more conventional microporous membrane separators, nonwovens have the advantage of low cost, low mass and high porosity; in addition, the fibrous mat provides good structural cohesion due to its intertwined fibers. Although most polymers used to make nonwoven battery separators have resulted in lower cell performance (lower ionic conductivity and higher resistance) than conventional microporous separators, polyvinylidene difluoride (PVDF) shows promising results because of its stability and affinity for electrolytes commonly employed in Li-ion cells. The best manner to produce nonwoven PVDF would employ a melt-blowing process, which is a well-developed, high-volume production technology. To be melt-blowable, polymer resins must have high melt-flow rates but commercial PVDF resins did not possess this property until recently. Nevertheless, researchers have successfully electrospun PVDF from a solution with the goal of exploring this promising polymer as a nonwoven battery separator. We are investigating the fundamental properties and characteristics of two novel melt-blowable PVDF grades (Kynar® 705 and Kynar® ADS II from Arkema) with the objective of elucidating their structure-property-process relationships and studying their performance as separators in Li-ion batteries. In this work, we will report on the physical, chemical, and electrochemical properties of nonwoven PVDF relevant to their use as battery separators.